The present invention pertains generally to methods and systems for using a hydrothermal reactor for the purposes of either waste destruction, energy generation, or production of chemicals. More specifically, the present invention pertains to methods and systems for the hydrothermal treatment of organics in a reactor when the organics contain or generate inorganic compounds such as salts or oxides during oxidation. The present invention is particularly, but not exclusively, useful as a method and system for using a reactor to accomplish the hydrothermal treatment of materials in a way which avoids the unwanted build-up of inorganic compounds in the reactor.
The present invention relates generally to the conversion of a broad spectrum of materials and especially to a method for the hydrothermal treatment of organics. Of particular importance to the present invention are organics which contain inorganic compounds such as salts or oxides or which will generate these inorganic compounds under supercritical temperature and pressure conditions, or at supercritical temperatures and elevated, yet subcritical pressures.
The process of wet oxidation has been used for the treatment of aqueous streams for over thirty (30) years. In general, a wet oxidation process involves the addition of an oxidizing agent, typically air or oxygen, to an aqueous stream at elevated temperatures and pressures. The resultant xe2x80x9ccombustionxe2x80x9d of organic or inorganic oxidizable materials occurs directly within the aqueous phase.
A wet oxidation process is typically characterized by operating pressures in the range of 30 bar to 250 bar (440 psia-3,630 psia) and operating temperatures in a range of one hundred fifty degrees Celsius to three hundred seventy degrees Celsius (150xc2x0 C.-370xc2x0 C.). Under these conditions, liquid and gas phases coexist for aqueous media. Since gas phase oxidation is quite slow at these temperatures, the reaction is primarily carried out in the liquid phase. To do this, the reactor operating pressure is typically maintained at or above the saturated water vapor pressure. This causes at least part of the water to be present in a liquid form. Even in the liquid phase, however, reaction times for substantial oxidation are on the order of one (1) hour. In many applications, reaction times of this length are unacceptable.
In addition to unacceptably long reaction times, the utility of conventional wet oxidation is limited by several factors. These include: the degree of oxidation attainable; an inability to adequately oxidize refractory compounds; and the lack of usefulness for power recovery due to the low temperature of the process. For these reasons, there has been considerable interest in extending wet oxidation to higher temperatures and pressures. For example, U.S. Pat. No. 2,944,396, which issued Jul. 12, 1960 to Barton et al., discloses a process wherein an additional second oxidation stage is accomplished after wet oxidation. In the Barton process, unoxidized volatile combustibles which accumulate in the vapor phase of the first stage wet oxidation reactor are sent to complete their oxidation in the second stage. This second stage is operated at temperatures above the critical temperature of water, about three hundred seventy four degrees Celsius (374xc2x0 C.).
A significant development in the field occurred with the issuance of U.S. Pat. No. 4,338,199, entitled xe2x80x9cProcessing Methods for the Oxidation of Organics In Supercritical Water,xe2x80x9d which issued to Modell on Jul. 6, 1982. Specifically, the Modell ""199 patent discloses a wet oxidation process which has now come to be widely known as supercritical water oxidation (xe2x80x9cSCWOxe2x80x9d). As the acronym SCWO implies, in some implementations of the SCWO process, oxidation occurs essentially entirely at conditions which are supercritical in both temperature (greater than 374xc2x0 C.) and pressure (greater than about 3,200 psi or 220 bar). Importantly, SCWO has been shown to give rapid and complete oxidation of virtually any organic compound in a matter of seconds at temperatures between five hundred degrees and six hundred fifty degrees Celsius (500xc2x0 C.-650xc2x0 C.) and at pressures around 250 bar. During this oxidation, carbon and hydrogen in the oxidized material form the conventional combustion products, namely carbon dioxide (xe2x80x9cCO2xe2x80x9d) and water. When chlorinated hydrocarbons are involved, however, they give rise to hydrochloric acid (xe2x80x9cHClxe2x80x9d), which will react with available cations to form chloride salts. Due to the corrosive effect of HCl, it may be necessary to intentionally add alkali to the reactor to avoid high concentrations of hydrochloric acid in the reactor. This is especially important in the cooldown equipment following the reactor. In a different reaction, when sulfur oxidation is involved, the final product in SCWO is a sulfate anion. This is in contrast to standard, dry combustion, in which gaseous sulfur dioxide (xe2x80x9cSO2xe2x80x9d) is formed and must generally be treated before being released into the atmosphere. As in the case of chloride, alkali may be intentionally added to avoid high concentrations of corrosive sulfuric acid. Similarly, the product of phosphorus oxidation is a phosphate anion.
At typical SCWO reactor conditions, densities are around 0.1 g/cc. Thus, water molecules are considerably farther apart than they are in water at standard temperatures and pressures (STP). Also, hydrogen bonding, a short-range phenomenon, is almost entirely disrupted, and the water molecules lose the ordering that is responsible for many of the characteristic properties of water at STP. In particular, the solubility behavior of water under SCWO conditions is closer to that of high pressure steam than to water at STP. Further, at typical SCWO conditions, smaller polar and nonpolar organic compounds, having relatively high volatility, will exist as vapors and are completely miscible with supercritical water. It also happens that gases such as nitrogen (N2), oxygen (O2), and carbon dioxide (CO2) show similar complete miscibility in supercritical water. The loss of bulk polarity in supercritical water also significantly decreases the solubility of salts. The lack of solubility of salts in supercritical water causes the salts to precipitate as solids and deposit on process surfaces causing fouling of heat transfer surfaces and blockage of the process flow.
A process related to SCWO known as supercritical temperature water oxidation (xe2x80x9cSTWOxe2x80x9d) can provide similar oxidation effectiveness for certain feedstocks but at lower pressure. This process has been described in U.S. Pat. No. 5,106,513, entitled xe2x80x9cProcess for Oxidation of Materials in Water at Supercritical Temperatures and Subcritical Pressures,xe2x80x9d which issued to Hong on Apr. 21, 1992, and utilizes temperatures in the range of six hundred degrees Celsius (600xc2x0 C.) and pressures between 25 bar to 220 bar. On the other hand, for the treatment of some feedstocks, the combination of temperatures in the range of four hundred degrees Celsius to five hundred degrees Celsius (400xc2x0 C.-500xc2x0 C.) and pressures of up to 1,000 bar (15,000 psi) have proven useful to keep certain inorganic materials from precipitating out of solution (Buelow, S. J., xe2x80x9cReduction of Nitrate Salts Under Hydrothermal Conditions,xe2x80x9d Proceedings of the 12th International Conference on the Properties of Water and Steam, ASME, Orlando, Fla., September, 1994).
The various processes for oxidation in an aqueous matrix (e.g. SCWO and STWO) are referred to collectively as hydrothermal oxidation, if carried out at temperatures between about three hundred seventy-four degrees Celsius to eight hundred degrees Celsius (374xc2x0 C.-800xc2x0 C.), and pressures between about 25 bar to 1,000 bar. Similar considerations of reaction rate, solids handling, and corrosion also apply to the related process of hydrothermal reforming, in which an oxidizer is largely or entirely excluded from the system in order to form products which are not fully oxidized. The processes of hydrothermal oxidation and hydrothermal reforming will hereinafter be jointly referred to as xe2x80x9chydrothermal treatment.xe2x80x9d
A key issue pertaining to hydrothermal treatment processes is the means by which feed streams containing or generating sticky solids are handled. It is well-known that such feed streams can result in the accumulation of solids that will eventually plug the process equipment. Sticky solids are generally comprised of salts, such as halides, sulfates, carbonates, and phosphates. One of the earliest designs for handling such solids on a continuous basis is disclosed in U.S. Pat. No. 4,822,497, entitled xe2x80x9cMethod for Solids Separation in a Wet Oxidation Type Process,xe2x80x9d which issued to Hong et al. In general, and in line with the disclosure of the ""497 patent, the reaction in a hydrothermal treatment process is carried out in a vertically oriented vessel reactor. Solids form in the reactor as the reaction proceeds and these solids are projected to fall into a cooler brine zone that is maintained at the bottom of the reactor. In the brine zone, the sticky solids re-dissolve and may be continually drawn off in the brine from the reactor. Solids separation from the process stream is achieved because only the fraction of the process stream that is necessary for solids dissolution and transport is withdrawn as brine. The balance of the process stream, which is frequently the largest portion, is caused to reverse flow in an upward direction within the reactor. The process stream, less the solids, is then withdrawn from the top section of the reactor. By this means, it becomes possible to recover a hot, nearly solids-free stream from the process. To minimize entrainment of solid particles in the upward flow within the reactor, the velocity is kept to a low value by using a large cross-section reactor vessel. Experience has shown that while a large fraction of the sticky solids is transferred into the brine zone, a certain portion also adheres to the vessel walls, eventually necessitating an online or off-line cleaning procedure.
Heretofore, mechanical scrapers which are movable relative to the vessel wall during the hydrothermal process have been suggested to reduce the buildup of solids on the vessel wall. For example, U.S. Pat. No. 5,100,560, entitled xe2x80x9cApparatus and Method for Supercritical Water Oxidation,xe2x80x9d which issued to Huang discloses a scraper that is rotatable about the axis of the reactor vessel for dislodging precipitated solids from the wall of a reactor vessel. Unfortunately, simple rotatable scrapers have enjoyed limited success to date. One reason for this lack of success is that solids often buildup on the scrapers preventing the scrapers from rotating and resulting in the reactor vessel becoming plugged.
In light of the above, it is an object of the present invention to provide a system and method for hydrothermal treatment which continuously and reliably handles waste streams containing or generating significant quantities of sticky solids while minimizing the need to take the reactor vessel off-line for cleanout. Another object of the present invention is to provide a system and method for accomplishing hydrothermal treatment in a continuous online process wherein the accumulation of solids on the vessel walls, scraper and scraper bars is minimized. Still another object of the present invention is to provide a system and method for accomplishing hydrothermal treatment which use scraper bars that include a mechanism for preventing solids from initially accumulating on the scraper bars. Yet another object of the present invention is to provide a system and method for accomplishing hydrothermal treatment which is easy to implement, simple to use, and cost effective.
In accordance with the present invention, a system for performing hydrothermal treatment at temperatures in a range from above three hundred seventy-four degrees Celsius (374xc2x0 C.) to about eight hundred degrees Celsius (800xc2x0 C.) and at pressures above about 25 bars, includes a substantially cylindrically shaped reactor vessel which forms a reactor chamber. The cylindrical vessel is formed with a wall having an inner surface and defines a longitudinal axis. Generally, the feed material is introduced into one end of the cylindrical reactor vessel and the reaction products are withdrawn from the other end of the cylindrical reactor vessel.
For the present invention, one or more elongated scraper bars are positioned in the reactor chamber substantially parallel to the longitudinal axis of the reactor vessel. An optional scraper may be used in conjunction with the scraper bars. For the present invention, the scraper is a hollow cylinder formed with an inside surface and an outside surface and defining a scraper axis. The scraper is positioned in the reactor vessel with the scraper axis substantially co-linear with the longitudinal axis of the cylindrical reactor vessel. As such, the outside surface of the scraper is positioned adjacent to the inner surface of the reactor vessel wall. The scraper may be solid or the scraper may contain holes which extend from the inside surface of the scraper to the outside surface of the scraper. A mechanism is provided to rotate the scraper about the longitudinal axis of the reactor vessel. The rotation of the scraper about the longitudinal axis results in a relative movement between the outside surface of the scraper and the inner surface of the reactor vessel wall allowing the scraper to dislodge any material that builds up on the inner surface of the reactor vessel wall.
At least one scraper bar is positioned inside the reactor vessel between the scraper and the longitudinal axis. Each scraper bar is positioned adjacent to the inside surface of the scraper. In one embodiment of the present invention, each scraper bar is positioned adjacent to the rotating scraper and held stationary with respect to the reactor vessel. Each scraper bar is generally elongated and formed with a characteristic shape normal to the direction of elongation. For example, a scraper bar may be formed with a circular, triangular or blade shape normal to the direction of elongation. Preferably, the scraper bar is oriented in the reactor chamber with the direction of elongation parallel to the longitudinal axis of the reactor vessel. In another embodiment, the scraper bar is positioned adjacent to the scraper and is rotatable about a scraper bar axis that passes through the scraper bar. Preferably, the scraper bar axis is substantially parallel to the direction of elongation of the scraper bar.
In yet another embodiment of the present invention, at least two scraper bars are positioned in the reactor chamber adjacent to the rotatable scraper. In this embodiment, each scraper bar is capable of nesting with another scraper bar. Preferably, each scraper bar is elongated and is formed with a leading edge and a trailing edge. As the scraper rotates, each point on the inside surface of the scraper first passes by the leading edge of the scraper bar and, subsequently passes by the trailing edge of the scraper bar. A separate arm is provided for each scraper bar. Each arm extends radially from a first end that is positioned on the longitudinal axis of the reactor chamber to a second end that is attached to the scraper bar to thereby allow each scraper bar to be independently rotated about the longitudinal axis of the reactor chamber. Further, each scraper bar is oriented at an angle, xcex1, relative to it""s attached arm to thereby cause the distance between the leading edge of the scraper bar and the inside surface of the scraper to be less than the distance between the trailing edge of the scraper bar and the inside surface of the scraper. A mechanism is provided to independently rotate each arm about the longitudinal axis of the reactor vessel to thereby allow for relative movement between adjacent scraper bars. Specifically, the trailing edge of one scraper bar can be passed by the leading edge of a second scraper bar to dislodge any solids that have accumulated on either scraper bar.
In still another embodiment of the present invention, scraper bars that are formed with internal cooling channels are positioned adjacent to a rotatable scraper. A mechanism is provided to circulate a coolant through the internal cooling channels, and thereby cool each scraper bar and the fluid that immediately surrounds each scraper bar. When the temperature of the fluid that surrounds each scraper bar is maintained below a critical temperature, inorganic material present in the fluid will not precipitate, and conversely, any precipitated solids will dissolve upon contact with the cooler fluid. In this manner, solids will be prevented from initially accumulating on the scraper bars.
In yet another embodiment of the present invention, scraper bars that are formed with purge holes located on the exterior surface of each scraper bar are positioned adjacent to a rotatable scraper. Internal fluid channels are provided within each scraper bar in fluid communication with the purge holes. A mechanism is provided to pass a purging fluid through the internal fluid channels for release into the reactor chamber through the purge holes. The purge holes are sized and configured to provide the exterior surface of each scraper bar with a jacket of purging fluid thereby preventing inorganic material from accumulating on the scraper bars.
In still another embodiment, a scraper is not used, and one or more scraper bars is positioned in the reactor chamber adjacent to the inner surface of the reactor vessel wall. A mechanism is provided to rotate the scraper bars about the longitudinal axis of the reactor vessel, thereby causing the scraper bars to move relative to the wall of the reactor vessel. In this manner, scraper bars similar to the nesting scrapper bars described above can be used without a rotatable scraper. In this embodiment, a mechanism to rotate the scraper bars as a group about the longitudinal axis is provided for scraping the inner wall of the reactor vessel. Additionally, a mechanism is provided for independently rotating each scraper bar relative to adjacent scraper bars, to allow the removal of solids buildup from the scraper bars.